/Engine/Source/Runtime/Core/Public/Math/MathFwd.h
Note that these conventions differ from quaternion axis/angle. UE Quat always considers a positive angle to be a left-handed rotation, whereas Rotator treats yaw as left-handed but pitch and roll as right-handed.
template<typename T> struct TRotator
T
Pitch
Rotation around the right axis (around Y axis), Looking up and down (0=Straight Ahead, +Up, -Down)
T
Roll
Rotation around the forward axis (around X axis), Tilting your head, (0=Straight, +Clockwise, -CCW)
T
Yaw
Rotation around the up axis (around Z axis), Turning around (0=Forward, +Right, -Left)